1. Field of the Invention
The present invention relates to a unidirectional surface acoustic wave transducer using Lithium Tetraborate (Li2B4O7) single crystals for a SAW (Surface Acoustic Wave) transversal filter.
2. Description of the Related Art
Recently, a SAW (Surface Acoustic wave) filter is mainly used in a mobile communication terminal. Particularly, in case of a mobile communication terminal using a CDMA (Code Division Multiple Access) system, the SAW filter is indispensably used as an intermediate frequency (IF) block filter. Further, a newly developed W-CDMA (Wideband CDMA) system requires the SAW filter as its essential device. As is known well, the SAW filters are commonly divided into two types, i.e., a resonator type and a transversal type. Since the linearity of phase is an important factor in the CDMA mobile communication terminal, in order to satisfy this requirement, the SAW transversal filter is mainly used.
As a conventional SAW transversal filter, a structure using a unidirectional surface acoustic wave transducer in the EWC (Electrode Width Control) type employing a piezoelectric substrate made of a crystal is known. (See U.S. Pat. No. 5,905,324) FIG. 2 is a cross-sectional view of this unidirectional surface acoustic wave transducer. The unidirectional surface acoustic wave transducer of FIG. 2 comprises three electrodes, i.e., an electrode finger Pa+, an electrode finger Paxe2x88x92 and an electrode Pr. The electrode Pa+ is positive and has a width of xcex/8. Herein, xcex represents a wavelength of the surface acoustic wave. The electrode Paxe2x88x92 and the electrode Pr are spaced from the electrode Pa+ by distances of respectively xcex/4 and 3xcex/8. As shown in FIG. 2, the electrode Pa and the electrode Pr are respectively disposed on each of both sides from the electrode Pa+. Herein, the distance between the electrode Pa+ and the electrode Paxe2x88x92, or the distance between the electrode Pa+ and the electrode Pr denotes a distance from the center of the electrode Pa+ to the center of the electrode Paxe2x88x92 or the center of the electrode Pr. The electrode Paxe2x88x92 and the electrode Pr are negative, and have widths of xcex/8 and xcex/4, respectively. The electrodes Pa+ and the electrode Paxe2x88x92 are excitation electrodes and the electrode Pr is a reflection electrode. The excitation electrode Paxe2x88x92 and the reflection electrode Pr may be respectively disposed on opposite sides from the excitation electrode Pa+.
It is known that the piezoelectric substrate is made of Lithium Tetraborate (Li2B4O7). Hereinafter, the piezoelectric substrate made of Lithium Tetraborate (Li2B4O7) is referred to as a LBO substrate. In case that a cutting angle of the LBO substrate and a propagation direction of the surface acoustic wave on the LBO substrate are (45xc2x0, 90xc2x0, 90xc2x0) by Euler""s angle or their equivalent degrees, the LBO substrate has a large electromechanical coupling coefficient k2, thereby being usable in a wide frequency band and having an excellent temperature stability. Therefore, the LBO substrate is suitable to a SAW filter of a W-CDMA type mobile communication terminal.
However, in the SAW transversal filter using a unidirectional surface acoustic wave transducer, in order to satisfy filter characteristics with symmetry on the center frequency, the phase difference (xcex94xcex8) between a reflection center and an excitation center needs to be 45xc2x0. However, since the phase difference (xcex94xcex8) of the EWC structure using the crystal substrate is approximately 49xc2x0, the crystal substrate cannot have good filter characteristics. Further, it will be expected that the LBO substrate has the same problem.
Therefore, the present invention has been made in view of the above problems, and it is an object of the present invention to provide a unidirectional surface acoustic wave transducer for a SAW transversal filter, which uses a piezoelectric substrate made of Lithium Tetraborate (Li2B4O7) and has a phase difference (xcex94xcex8) of approximately 45xc2x0, thereby improving its filter characteristics.
In accordance with one aspect of the present invention, the above and other objects can be accomplished by the provision of a unidirectional surface acoustic wave transducer for propagating surface acoustic waves in one direction, formed on a piezoelectric made of Lithium Tetraborate (Li2B4O7) single crystals and comprising metal electrodes mainly made of aluminum (Al) in a designated thickness H, the metal electrodes comprising: a positive first electrode finger having a width of xcex/8 (wherein, xcex is a wavelength of the surface acoustic wave); a negative second electrode finger having a width of xcex/8, spaced from the first electrode finger at a distance of xcex/4, and formed on a side of the first electrode finger; and a negative third electrode finger having a designated width M, spaced from the first electrode finger at a distance of 3xcex/8, and formed on the other side of the first electrode finger, wherein a cutting angle on the piezoelectric substrate and a propagation direction of the surface acoustic wave on the piezoelectric substrate are in the range of 45xc2x0xc2x11xc2x0, 90xc2x0xc2x11xc2x0, and 90xc2x0xc2x11xc2x0 by Euler""s angle, or in their equivalent range, and a relation between a relative electrode thickness H/xcex(%) obtained by dividing the thickness H of the metal electrode by the wavelength xcex of the surface acoustic wave and a relative electrode finger width M/xcex obtained by dividing the width M of the third electrode finger by the wavelength xcex of the surface acoustic wave is defined by the following formula
M/xcex=0.0075xc3x97(H/xcex(%))2xe2x88x920.0624xc3x97(H/xcex(%))+0.3652xc2x10.02.xe2x80x83xe2x80x83Formula (1) 
Preferably, the relative electrode thickness H/xcex(%) obtained by dividing the thickness H of the metal electrode by the wavelength xcex of the surface acoustic wave may be in the range of 1% to 2%.
Further, preferably, the relative electrode thickness H/xcex(%) obtained by dividing the thickness H of the metal electrode by the wavelength xcex of the surface acoustic wave and the relative electrode finger width M/xcex obtained by dividing the width M of the third electrode finger by the wavelength xcex of the surface acoustic wave may be determined by X and Y values of an intersection point between a first curve defined by the formula (1) and a second curve defined by the following formula (2)
M/xcex=xe2x88x920.0103xc3x97(H/xcex(%))2+0.0653xc3x97(H/xcex(%))+0.2141xc2x10.02.xe2x80x83xe2x80x83Formula (2) 
In accordance with another aspect of the present invention, there is provided a unidirectional surface acoustic wave transducer for propagating surface acoustic waves in one direction, formed on a piezoelectric made of Lithium Tetraborate (Li2B4O7) single crystals and comprising metal electrodes mainly made of aluminum (Al) in a designated thickness H, the metal electrodes comprising: a positive first electrode having a width of xcex/8 (wherein, xcex is a wavelength of the surface acoustic wave); a negative second electrode finger having a width of xcex/8, spaced from the first electrode finger at a distance of xcex/4, and formed on a side of the first electrode finger; and a negative third electrode finger having a designated width M, spaced from the first electrode finger at a distance of 3xcex/8, and formed on the other side of the first electrode finger, wherein a cutting angle on the piezoelectric substrate and a propagation direction of the surface acoustic wave on the piezoelectric substrate are in the range of 45xc2x0xc2x11xc2x0, 90xc2x0xc2x11xc2x0, and 90xc2x0xc2x11xc2x0 by Euler""s angle, or in their equivalent range, and a relative electrode thickness H/xcex(%) obtained by dividing the thickness H of the metal electrode by the wavelength xcex of the surface acoustic wave is in the range of 1% to 2%, and a relative electrode finger width M/xcex obtained by dividing the width M of the third electrode finger by the wavelength xcex of the surface acoustic wave is in the range of 0.26 to 0.33.